Titanium dioxide coating method

ABSTRACT

A titanium dioxide coating method is disclosed. An electrolyte containing Ti 3+ , an oxidant, and at least one of NO 3   −  and NO 2   −  is provided for an electrodeposition device, wherein the oxidant is configured for essentially oxidizing Ti 3+  into Ti 4+ . A substrate is immersed into the electrolyte and electrically connected to the electrodeposition device. A cathodic current is applied to the substrate via the electrodeposition device for reduction of NO 2   −  or NO 3   − . A titanium dioxide film is thus formed on the surface of the substrate. The thickness, porosity, and morphology of the titanium dioxide film can be controlled by varying the electroplating parameters, and relatively uniform deposits on various substrates of complex shapes can be obtained by use of low cost instruments. The resultant structure of Ti 4+  species oxidized from Ti 3+  by the oxidant can be used to control the deposition rate of TiO 2 .

RELATED APPLICATIONS

This application is a Continuation-in-part of co-pending applicationSer. No. 12/505,936 filed on Jul. 20, 2009.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a titanium dioxide coating method andthe electrolyte used therein, and more particularly to anelectrodeposition method for coating titanium dioxide and theelectrolyte used therein.

2. Description of the Prior Art

Titanium dioxide, also known as titania, is widely recognized as animportant electrode material in semiconductor photo-electrochemistry.Among the three main crystalline phases: anatase, rutile, and brookiteTiO₂, the anatase form (A-TiO₂) is the most popular photo-electrodebecause the lowest unoccupied molecular orbital of dyes, such as N719,is very close to the conduction band of A-TiO₂.

In addition, A-TiO₂ generally shows relatively high reactivity andchemical stability under ultraviolet light excitation for water and airpurifications, photocatalysts, gas sensors, electrochromic devices, andso on, further emphasizing its practical importance.

Several techniques were proposed for fabricating TiO₂, such as sol-gel,chemical vapor deposition, hydrothermal, electrospinning, anodizing, andelectrodeposition.

Among these methods, cathodic deposition of TiO₂ becomes attractivebecause electrochemical deposition provides the advantages ofcontrolling the thickness and morphology by varying the electroplatingparameters, relatively uniform deposits on complex shapes, and use oflow cost instrumentation.

Sotiropoulos et al. (Electrochimica Acta 51 (2006) 2076-2087) preparedTiO2 films from acidic aqueous solutions of TiOSO₄ and H₂O₂ by roomtemperature potentiostatic cathodic electrosynthesis. However,Sotiropoulos taught that TiOSO₄ was oxidized to Ti⁶⁺ by using a strongoxidant H₂O₂, which needs to be reduced to prepare the TiO₂ film.

Kim et al. (Electrochimica Acta 50 (2005) 2713-2718) taught a novelapproach using TiCl₃ or TiCl₄ as the precursors for theelectrodeposition of TiO₂ films. Kim mainly focused on the advantage inusing CTAB and the pH value of the solution is roughly 3 in all thecases (Kim, p. 2714 Experimental section 2, paragraph 2).

Both of Sotiropoulos and Kim did not achieve high yield of titaniumdioxide and it is now a current goal to develop a cathodic depositionmethod for coating titanium dioxide with higher yield in comparison withthe prior arts.

SUMMARY OF THE INVENTION

The present invention is directed to provide an electrolytic method forcoating titanium dioxide to gain the advantages of controlling thethickness, porosity, and morphology by varying the electroplatingparameters, relatively uniform deposits on various substrates of complexshapes, and use of low cost instrumentations.

The present invention is directed to a cathodic deposition method forcoating a titanium dioxide film with higher yield in comparison with theprior arts.

According to one embodiment, the present invention provides a titaniumdioxide coating method, which includes following steps. An electrolytecontaining Ti³⁺, an oxidant and at least one of NO₃ ⁻ and NO2⁻ isprovided for an electrodeposition device, wherein the oxidant isconfigured for essentially oxidizing Ti³⁺ into Ti⁴⁺. A substrate isimmersed into the electrolyte and electrically connected to theelectrodeposition device. A cathodic current from the electrodepositiondevice is applied to the substrate for reducing NO₂ ⁻ or NO₃ ⁻ togenerate extensive OH⁻ and to form titanium dioxide film on the surfaceof the substrate.

Other advantages of the present invention will become apparent from thefollowing descriptions taken in conjunction with the accompanyingdrawings wherein are set forth, by way of illustration and example,certain embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the accompanying advantages of thisinvention will become more readily appreciated as the same becomesbetter understood by reference to the following detailed description,when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 illustrates a flowchart of a titanium dioxide coating methodaccording to one embodiment of the present invention;

FIG. 2 illustrates LSV (linear sweep voltammetry) curves according toone embodiment of the present invention;

FIG. 3A illustrates first and second scans of LSV curves according toone embodiment of the present invention;

FIG. 3B illustrates the corresponding EQCM (electrochemical quartzcrystal microbalance) responses of the first and second scans of LSV inFIG. 3A according to one embodiment of the present invention;

FIG. 3C illustrates an enlarged view of curve 1 in FIG. 3B;

FIG. 3D illustrates a SEM image of titanium dioxide depth for 3 cyclesaccording to one embodiment of the present invention.

FIGS. 4A and 4B illustrate SEM (Scanning Electron Microscope) imagesaccording to one embodiment of the present invention;

FIGS. 4C and 4D illustrate TEM (Transmission Electron Microscope) imagesaccording to one embodiment of the present invention;

FIGS. 4E and 4F illustrate depth profiles of XPS (X-ray photoelectronspectra) according to one embodiment of the present invention;

FIG. 5A illustrates the LSV curves according to one embodiment of thepresent invention;

FIG. 5B illustrates the corresponding EQCM (electrochemical quartzcrystal microbalance) responses of the LSV curves in FIG. 5A accordingto one embodiment of the present invention; and

FIG. 5C illustrates the dependence of TiO₂ mass on the cycle number ofCV according to one embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a flowchart of a titanium dioxide coating methodincluding following steps. Beginning at step S1, an electrolyte with pHvalues ≦2 and containing Ti³⁺, an oxidant and at least one of NO₃ ⁻ andNO₂ ⁻. The Ti³⁺ is essentially oxidized into Ti⁴⁺ by the oxidant and NO₃⁻/NO₂ ⁻ is the OH⁻ provider. This electrolyte is provided for anelectrodeposition device. Next, at step S2, a substrate is then immersedinto the electrolyte and at step S3, the substrate is electricallyconnected to the electrodeposition device. At step S4, a cathodiccurrent is applied on the substrate via the electrodeposition device forreducing NO₂ ⁻ or NO₃ ⁻ to generate extensive OH⁻ for depositing TiO₂films on the surface of substrates. The cathodic current can be appliedby galvanostatic (constant dc current), potentiostatic (constantvoltage), potentiodynamic, or galvanodynamic methods, or in the pulsevoltage or pulse current modes.

In one preferred embodiment, an electrolyte with pH values <1 isprovided for titanium dioxide deposition. Ti³⁺ may be obtained fromdissolution of titanium, for example by dissolving with H₂O₂ andammonia.

The oxidants can be divided into two groups, strong and weak oxidants.When the weak oxidants are employed, Ti³⁺ can only be oxidized to Ti⁴⁺,even excess oxidants are added. When the strong oxidants are employed, astoichiometric ratio between Ti³⁺ and oxidants is required to oxidizeTi³⁺ to Ti⁴⁺ which cannot be further oxidized to Ti⁶⁺. Referring toTable 1, weak oxidants that essentially oxidize Ti³⁺ into Ti⁴⁺ areprovided and include without limitations to NO₃ ⁻, NO₂ ⁻, S₂O₈ ²⁻, ClO₄⁻, ClO⁻, BrO₄ ⁻, BrO⁻, IO₄ ⁻ or IO⁻. The strong stoichiometric oxidantsinclude without limitations to H₂O₂ or O₃.

TABLE 1 Oxidants that essentially oxidize Ti³⁺ into Ti⁴⁺ OxidantTi³⁺→Ti⁴⁺ ^(@)Ti⁴⁺→Ti⁶⁺ Color NO₃ ⁻ Yes No transparent NO₂ ⁻ Yes Notransparent *XO₄ ⁻ Yes No transparent to pale yellow^(%) *XO⁻ Yes Notransparent to pale yellow^(%) S₂O₈ ⁻ Yes No transparent to paleyellow^(%) H₂O₂ Yes^(#) Yes Tangerine O₃ Yes^(#) Yes Tangerine *Xrepresents Cl, Br, I ^(#)Stoichiometric ratio ^(@)excess oxidant^(%)turning pale yellow when excess oxidant is present

The continuous reduction of NO₂ ⁻ or NO₃ ⁻ to N₂ and NH₃ generatesextensive OH⁻, and effectively enhances the deposition of TiO₂ films onthe surface of substrates.

In one embodiment, a post annealing step is further performed afterforming the titanium dioxide film on the surface of the substrate,wherein the post annealing step is carried out at about 100-800° C.

The following descriptions of specific embodiments of the presentinvention have been presented for purposes of illustrations anddescription, and they are not intended to be exclusive or to limit thepresent invention to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteaching. It is intended that the scope of the invention can be definedby the Claims appended hereto and their equivalents.

TiO₂ particulates are cathodically deposited onto graphite substratesfrom an electrolyte bath containing 0.47 M HCl, 25 mM TiCl₃ and 75 mMNaNO₃ in an electrodeposition device according to an embodiment of thepresent invention. A pretreatment procedure of graphite substrates maybe performed and the detailed description thereof is herein omitted.

According to one embodiment of the present invention, the redox reactionbetween Ti³⁺ and NO₃ ⁻ during preparation of the deposition solution isherein disclosed. Nitrates, acting as the oxidizers, were reduced to NO₂(reddish-brown bubbles) when the transparent NaNO₃ solution was addedinto the purple TiCl₃ solution. Since NO₂ molecules are soluble inacidic aqueous media, they will automatically convert into NO₃ ⁻ and NO.This statement is supported by the observation that reddish-brownbubbles gradually disappear within 30-40 seconds and the purple TiCl₃solution in presence of Ti³⁺ is a colorless transparent solutionindicating the formation of TiO²⁺ (see equations 1 and 2)

Ti³⁺+NO³⁻→TiO²⁺+NO₂  (1)

3NO₂+H₂O→2HNO₃+NO  (2)

Curves 1-5 in FIG. 2 correspond to the i-E responses measured fromvarious electrolytes. As can be seen from curves 1 and 2, reductioncommences at potentials negative to −0.6 V and no gas evolution is foundat potentials positive to −0.6 V. However, a rapid generation of manybubbles is clearly observed when potentials are negative to −0.6 V,indicating H₂ evolution. On curves 3 and 4, reduction starts in the morepositive potential region, revealing the facile reduction of NaNO₂. Inaddition, minor gas evolution commences from 0.4 V to −0.4 V with a lowcurrent density, while gas evolution ceases in the potential range from−0.4 V to −1.2 V and occurs dramatically again at potentials behind −1.2V. The above results indicate that NO₂ ⁻ is responsible for thereduction in the more positive potential region with minor gasevolution, presumably due to the reduction of NO₂ ⁻ into N₂ molecules.Since gas evolution temporarily disappears in the potential range from−0.4 V to −1.2 V. This result suggests a further reduction of N₂ to NH₄⁺ in such a negative potential range (see equations 3 and 4).

2NO₂ ⁻+4H₂O+6e→N₂+8OH⁻  (3)

N₂+8H₂O+6e→2NH₄ ⁺+8OH⁻  (4)

On curve 5, gas evolves gently at about −0.1 V, disappears at ca. −0.4 Vand, dramatically evolves again at potentials negative to −1.2 V, whichcompletely follows the gas evolution-disappearance phenomena measuredfrom the solution containing NO₂ ⁻. Based on equations 1 and 2,reduction of NO₃ ⁻ in the designed deposition bath for generatingconcentrated OH⁻ at the vicinity of electrode surface is very similar tothe reduction of NO₂ ⁻ (see equation 5). Accordingly, reduction of NO₂ ⁻or NO₃ ⁻ is concluded to be an effective step in promoting thedeposition of TiO(OH)₂ (see equation 6). The TiO(OH)₂ is then dehyratedto form TiO₂ (see equation 7).

2NO₃ ⁻+6H₂O+10e→N₂+12OH⁻  (5)

TiO²⁺+2OH⁻+xH₂O→TiO(OH)₂.xH₂O  (6)

TiO(OH)₂.xH₂O→TiO₂+(x+1)H₂O  (7)

The mechanism proposed in this invention not only reasonably interpretsthe gas evolution/disappearance phenomena but also explains the slightincrease in bath pH after the deposition, which is different from theslight decrease in pH found in previous case of NO₃ ⁻ reduction. Basedon equations 3, 4, and 6, OH⁻ is mainly provided by the NO₂ ⁻ or NO₃ ⁻reduction and the consequent N₂ reduction, resulting in the generationof NH₄ ⁺. As a result, a slight increase in pH found in this formulatedsolution after TiO₂ deposition is reasonable because the OH⁻/ electronratios for the reduction of NO₂ ⁻, NO₃ ⁻, and N₂ are equal to 4/3, 6/5,and 4/3, respectively, which are larger than the proton/electron ratio(equal to 1) for oxygen evolution at the anode. Moreover, the depositionrate in this formulated solution is very fast, attributable to themassive generation of OH⁻, the catalytic reduction of NO₂ ⁻ and NO₃ ⁻ byTiO(OH)₂ and TiO₂, and the guarantee of TiO²⁺ formation via the redoxreaction between Ti³⁺ and oxidants such as NO₃ ⁻/NO₂ ⁻.

FIG. 3A illustrates the first and second scans of LSV (linear sweepvoltammetry) curves and FIG. 3B illustrates the corresponding EQCM(electrochemical quartz crystal microbalance) responses of the first andsecond scans of LSV measured from the designed solution in order toprecisely obtain the onset potential of deposition. A comparison of thei-E and mass-E responses indicates that there is always an incubationperiod for N₂ evolution in the positive potential range, e.g., from 0.2to −0.7 V and from 0.1 to −0.65 V for the first and second sweeps,respectively. Although in the incubation range, NO₂ ⁻ and NO₃ ⁻ start tobe reduced to N₂, no significant increase in mass is observed. Theslight weight gain in this potential region is probably due to the NO₂⁻/NO₃ ⁻ adsorption at the cathode. Based on the EQCM result, once thepotential is negative enough to generate/accumulate concentrated OH⁻,TiO²⁺ will combine with OH⁻ to form TiO₂ and an obvious weight gain isvisible behind this onset potential of deposition (−0.85 and −0.65 V forthe first and second scans, respectively). Also note the positive shiftin the onset potential of deposition during the second scan. Thisphenomenon is probably due to the electrocatalytic property of TiO(OH)₂and TiO₂ already deposited onto the graphite surface during the firstscan for NO₃ ⁻/NO₂ ⁻/N₂ reduction.

Referring to FIG. 3D, the present invention achieve ca. 20 μm (5.4, 7.4and 7.6 μm for 3 cycles). The dashed lines in FIG. 3D indicate theboundary between deposit and substrate as well as the boundaries of TiO₂deposits between each CV cycle, respectively. The catalytic effect ofTiO(OH)₂ and TiO₂ for the NO₃ ⁻, NO₂ ⁻, and N₂ reduction is also one ofthe main reasons why the present invention achieved a much higher yieldof titanium dioxide (in comparison to 4 μm for 20 cycles for Kim etal.). In addition, the usage of weak oxidants, such as NO₃ ⁻ and NO₂ ⁻even in excess, guarantees the formation of TiO²⁺, which is also one ofthe main reasons why the present invention achieved a much higher yieldof titanium dioxide.

The electrodes were cleaned in an ultrasonic DI water bath and driedunder a cool air flow after cathodic deposition. After cleaning anddrying, some electrodes were annealed at 400° C. in air for 1 hr. Themorphologies were examined by a FE-SEM (Field-Emission Scanning ElectronMicroscope, FE-SEM). The EQCM study was performed by an electrochemicalanalyzer, CHI 4051A in a one-compartment cell. The microstructure andSAED (selected area electron diffraction, SAED) patterns of as-depositedand annealed TiO₂ deposits were observed through a TEM (FEI E.O TecnaiF20 G2). The depth profiles of Ti and O were measured by an X-rayphotoelectron spectrometer (XPS, ULVAC-PHI Quantera SXM), employed Almonochromator (hv=1486.69 eV) irradiation as the photosource.

It is favorable to prepare porous A-TiO₂ films by combining cathodicdeposition from this designed solution with lower pH value andpost-deposition annealing. As illustrated in FIGS. 4A and 4B, TiO₂ filmsbefore and after annealing are porous and the particle size is roughlyestimated to be 60-100 nm. The porous nature of TiO₂ films prepared inthis invention is probably due to the extensive tiny bubble evolutionduring the deposition. The particulates are considered as aggregates ofTiO₂ primary particles.

The average size for as-deposited TiO₂ primary particles is about 6 nm,which is enlarged by post-deposition annealing (ca. 10 nm for TiO₂annealed at 400° C.) from FIGS. 4C and 4D. The lattice clearly visiblein FIG. 4D and the diffraction rings in its inset indicate the anatasestructure which is transformed from the amorphous, as-deposited TiO₂ bypost-deposition annealing. FIGS. 4E and 4F illustrate the depth profilesof Ti, O, and C for as-deposited and annealed samples. Clearly, theatomic ratio of Ti/O is approximately constant (ca. 1/2) within thewhole oxide matrix.

These results confirm the formation of TiO₂ in the as-prepared andannealed films. Accordingly, combining cathodic deposition from thisdesigned solution and post-deposition annealing is favorable forpreparation of porous A-TiO₂ films.

The aforementioned embodiment exemplified the reaction from theelectrolyte solution containing Ti³⁺⁺ NO₃ ⁻; however, the redox reactionbetween Ti³⁺ and NO₂ ⁻ in an electrolyte solution can be used fordepositing titanium dioxide films, too (See Equations 3, 4, 6, and 8).

6Ti³⁺+2NO₂₃₁ +2H₂O→6TiO²⁺+N₂+4H⁺  (8)

FIGS. 5A and 5B show the typical LSV and Δm-E curves measured at 25 mVs⁻¹ from 0 to −1.6 V (vs. Ag/AgCl) in diluted baths A and B,respectively. Bath A is defined as a deposition solution containing 30mM H₂O₂, 60 mM TiCl₃, and 75 mM NaNO₃. Bath B is defined as a depositionsolution containing 60 mM TiCl₃ and 135 mM NaNO₃. In FIG. 5A, the onsetpotential of reduction on both i-E curves is the same, −0.47 V, which isreasonably due to the same reaction, NO₃ ⁻ reduction on the EQCMelectrode. The reduction currents on curve 1 are always higher than thaton curve 2 at any specified potentials negative to −0.47 V although theconcentration of NO₃ in both baths should be the same under theassumption that most NO₂ gases generated in bath B are not dissolved inthe deposition bath. Accordingly, the formation of certain Ti^(4′)hydroxyl species (e.g.,

in bath A is favorable for the NO₃ ⁻ reduction.

Referring to FIG. 5B, the mass of TiO₂ increases sharply from 0 to 70 ngin the potential region between −0.71 and −0.8 V and then, a gradualincrease to 145 ng at potentials negative to −0.8 V on curve 1. On curve2, significant increase in mass commences at ca. −0.68 V and then, ashoulder is found between −0.68 and −0.9 V. After that, a sharp increasein mass occurs from −0.9 to −1.0 V and a gradual increase from 70 to 130ng at potentials negative to −1.0 V. Clearly, the TiO₂ deposition ratein bath A is obviously higher than that in bath B, attributable to theformation of Ti⁴⁺ hydroxyl species containing bridged OH groups in thesolution. Such Ti⁴⁺ hydroxyl species (with olation) need fewer OH⁻ toform the polymeric oxy-hydroxyl Ti precipitates which will be convertedto TiO₂ through dehydration. Accordingly, the formation of Ti⁴⁺ hydroxyldimmers containing bridged OH groups favors the cathodic deposition ofTiO₂.

Referring to FIG. 5C, Lines 1 and 2 show the dependence of TiO₂ mass onthe cycle number of CV between 0 and −1.6 V from baths A and B,respectively. Clearly, the dependence of TiO₂ mass on the cycle numberof CV from both deposition baths is linear. However, the slope of curve1 is obviously higher than that of curve 2, revealing that thedeposition solution containing H₂O₂ is more favorable for the cathodicdeposition of TiO₂ in comparison with that containing NO₃ ⁻ only. Hence,the resultant structure of Ti⁴⁺ species oxidized from Ti³⁺ by theoxidant determines the deposition rate of TiO₂.

To sum up, a titanium dioxide coating method according to the presentinvention includes a cathodic deposition using an electrolytic solutioncontaining Ti³⁺, an oxidant, and at least one of NO₃ ⁻ and NO₂ ⁻, and apost-deposition annealing process, which is favorable for preparingporous A-TiO₂ films. The redox reaction between Ti³⁺ and oxidant to formTi⁴⁺ prior to cathodic deposition effectively promotes the TiO₂deposition. The resultant structure of Ti⁴⁺ species oxidized from Ti³⁺by the oxidant determines the deposition rate of TiO₂. The continuousreduction of NO₂ ⁻ or NO₃ ⁻ to N₂ and NH₃ generates extensive OH⁻ andeffectively enhances the deposition of TiO₂ for forming a TiO₂ film atthe substrate surface.

The porous, anatase structure of annealed TiO₂, examined by FE-SEM, TEM,and SAED analyses is expected to be good for the dye-sensitized solarcell (DSSC) application. In addition, A-TiO₂ may be applicable for waterand air purifications, photocatalysts, gas sensors, electrochromicdevices, and so on.

While the invention is susceptible to various modifications andalternative forms, a specific example thereof has been shown in thedrawings and is herein described in detail. It should be understood,however, that the invention is not to be limited to the particular formdisclosed, but to the contrary, the invention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the appended claims.

1. A titanium dioxide coating method comprising: providing anelectrolyte with a pH value ≦2 and containing Ti³⁺, an oxidant, and atleast one of NO₃ ⁻ and NO₂ ⁻ for an electrodeposition device, whereinthe oxidant is configured for essentially oxidizing Ti³⁺ into Ti⁴⁺;immersing a substrate into the electrolyte; electrically connecting thesubstrate to the electrodeposition device; and applying a cathodiccurrent to the substrate via the electrodeposition device whereby NO₃ ⁻or NO₂ ⁻ is reduced to generate extensive OH⁻ for forming a titaniumdioxide film on the surface of the substrate.
 2. The method as claimedin claim 1, wherein the oxidant is a weak oxidant which is unable tofurther oxidize Ti⁴⁺ to Ti⁶⁺.
 3. The method as claimed in claim 2,wherein the weak oxidant comprises NO₃ ⁻ or NO₂ ⁻.
 4. The method asclaimed in claim 2, wherein the weak oxidant comprises S₂O₈ ²⁻, ClO₄ ⁻,ClO⁻, BrO₄ ⁻, BrO⁻, IO₄ ⁻ or IO⁻.
 5. The method as claimed in claim 2,wherein a ratio of Ti³⁺ to the weak oxidant is equal to/above thestoichiometric ratio.
 6. The method as claimed in claim 1, wherein theoxidant is a strong oxidant in a stoichiometric ratio to Ti³⁺.
 7. Themethod as claimed in claim 6, wherein the strong oxidant comprises H₂O₂or O₃.
 8. The method as claimed in claim 1, wherein the pH value of theelectrolyte is less than
 1. 9. The method as claimed in claim 1 furthercomprising a post annealing step after forming the titanium dioxidefilm.
 10. The method as claimed in claim 9, wherein the post annealingstep is carried out at about 100-800° C.
 11. The method as claimed inclaim 1, wherein the cathodic current is applied by galvanostatic(constant dc current), potentiostatic (constant voltage),potentiodynamic, or galvanodynamic methods, or in the pulse voltage orpulse current modes.
 12. The method as claimed in claim 1, wherein OH⁻is generated by reduction of NO₃ ⁻ or NO₂ ⁻ at the cathode.
 13. Themethod as claimed in claim 12, wherein TiO(OH)₂ is generated from areaction between Ti⁴⁺ and OH⁻ and then dehydrated to form TiO₂.
 14. Themethod as claimed in claim 13, wherein the generation of OH⁻ by NO₃ ⁻ orNO₂ ⁻ reduction at the cathode is catalyzed by TiO(OH)₂ and TiO₂. 15.The method as claimed in claim 1, wherein the Ti³⁺ is obtained fromdissolution of titanium.